Method for growing single crystal silicon carbide

ABSTRACT

A method and apparatus for axially growing single crystal silicon carbide is provided. Utilizing the system, silicon carbide can be grown with a dislocation density of less than 10 4  per square centimeter, a micropipe density of less than 10 per square centimeter, and a secondary phase inclusion density of less than 10 per cubic centimeter. As disclosed, a SiC source and a SiC seed crystal of the desired polytype are co-located within a crucible, the growth zone being defined by the substantially parallel surfaces of the source and the seed in combination with the sidewalls of the crucible. Prior to reaching the growth temperature, the crucible is evacuated and sealed, either directly or through the use of a secondary container housing the crucible. The crucible is comprised of tantalum or niobium that has been specially treated. As a result of the treatment, the inner surfaces of the crucible exhibit a depth variable composition of Ta—Si—C or Nb—Si—C that is no longer capable of absorbing SiC vapors, thus allowing the vapor-phase composition within the crucible to be close to the SiC—Si system with the partial pressure of Si-vapor slightly higher than that in the SiC—Si system.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/355,561, filed Jul. 20, 1999, which claimspriority from PCT Application Serial No. PCT/RU97/00005, filed Jan. 22,1997.

FIELD OF THE INVENTION

[0002] The present invention relates generally to the generation ofmonocrystalline materials and, more particularly, to a method andapparatus for growing monocrystalline silicon carbide.

BACKGROUND OF THE INVENTION

[0003] Silicon carbide (SiC) is wide-band-gap semiconductor materialthat has a number of characteristics that make it an ideal candidate fora variety of semiconductor applications including, but not limited to,light sources, power diodes, field-effect transistors, and photodiodes.The ability to realize the benefits offered by SiC is largely controlledby the purity of the material as well as its structural characteristics.

[0004] The methods most commonly used in producing SiC single crystalsare sublimation techniques based on the Lely method, this methodutilizing vapor-phase crystallization of evaporated solid siliconcarbide. (See, for example, U.S. Pat. Nos. 2,854,364 and 4,866,005). Asshown by Karpov et al. in an article entitled Excess Phase FormationDuring Sublimation Growth of Silicon Carbide, 6th Int. Conf. on SiliconCarbide, Kyoto, Japan, 74-75 (September 1995), in order to achieve SiCmonocrystalline growth from vapor without forming secondary-phaseinclusions, the external silicon (Si) flux on the growing surface mustexceed the carbon (C) flux. The ability to achieve the desired excesssilicon flux depends on the temperature of the growing surface and, inthe case of sublimation techniques, the composition of the vaporadjacent to the growth surface.

[0005] As silicon molecules have the maximum concentration in thegaseous phase, any drift of the substance from the growth zone willresult in the vapor phase within the growth zone being depleted ofsilicon and enriched with carbon. Excessive carbon in the growth zoneleads to source graphitization, crystal quality degradation, andeventually the discontinuation of the growth process. On the other hand,excessive silicon in the growth zone may result both in the formation ofdefects on the growing surface of the SiC crystal, primarily due to theprecipitation of excess silicon drops, and in the generation ofpolytypes differing from the seed polytype. Accordingly, it has beenestablished that the best characteristics for the as-grown SiC singlecrystal are achieved when the vapor composition in the growth zone isshifted the towards the vapor phase corresponding to the SiC—Si system.

[0006] U.S. Pat. No. 2,854,364 discloses locating SiC powder with a massof more than three times the mass of the single crystal to be grown inthe growth zone in order to maintain a relatively constant vapor phasecomposition, the powder serving as a source of silicon carbide vapors.As disclosed, the drift of SiC vapors into the space outside the growthzone is balanced by the generation of SiC vapors from the SiC powder.The duration of the stable growth process is limited by the quantity ofSiC powder located in the growth zone. Once the source of SiC vaporsbecomes depleted, the vapor composition shifts to the non-advantageousSiC—C system.

[0007] In U.S. Pat. No. 4,866,005 a technique is disclosed thatcontinuously feeds small portions of SiC powder into a temperature zoneof the growth chamber. Although this technique does allow a SiC—Sisystem to be maintained indefinitely, it is an inefficient process dueto the SiC material consumed in addition to the SiC source as well asthe growth zone geometry. As disclosed, the evaporating surface of theSiC vapor source is approximately 10 centimeters from the growingsurface of the seed crystal, a distance that far exceeds the Si, Si₂C,SiC₂ molecular track length at the working pressure in the growth zone.

[0008] U.S. Pat. No. 4,147,572 discloses a growth technique in which theevaporating surface of the SiC source and the growth surface of the SiCseed crystal are arranged in parallel at a distance that is less than 20percent of the maximum linear dimension of the source. The singlecrystals are grown in a graphite crucible in an inert gas atmosphere attemperatures of 1600 to 2000° C. with an axial thermal gradient of 5 to200° C. per centimeter. This technique is limited to relatively smallcrystals, typically less than 1 millimeter thick, due to a sharp drop inthe growth rate as the crystallization time increases. The change ingrowth rate is due to the silicon at the edge of the growth zone beingvolatilized, thereby causing excessive carbon to be released from theevaporating surface of the SiC source and the growing surface of thegrown crystal. Single crystals obtained by this technique show defectssuch as secondary-phase inclusions (predominantly, graphite), micropipeswith a density of more than 100 per square centimeter, and dislocationsof at least 10⁴ per square centimeter. These crystals also haverelatively high concentrations of residual impurities such as boron,oxygen, etc.

[0009] In an article by D. Hofmann et al. entitled The Use of TantalumContainer Material for Quality Improvement of SiC Crystals Grown by theSublimation Technique, 6th Int. Conf. on Silicon Carbide, Kyoto, Japan,15 (September 1995), it was shown that the inclusion of tantalum (Ta)during the sublimation growth of monocrystalline SiC resulted in thevapor medium produced in the growth zone being close to the SiC—Sisystem. The favorable aspects were found to occur both in an inert gasatmosphere and in vacuum. Unfortunately it was also found that duringthe early stages of growth, secondary-phase inclusions of tantalum orits compounds were formed. An increased concentration of dissolvedtantalum in the monocrystalline SiC was also noted. Lastly, due to thecarbon enrichment of the vapor phase that results from silicon driftingoutside of the growth zone, carbon dust was embedded into the growingcrystal, further reducing the quality of the growing crystal whilesimultaneously decreasing the transferal efficiency of source materialto the growing crystal.

[0010] Another problem associated with the use of a Ta container asdisclosed in the previously cited article arises during the initialstage of the growth process when the silicon vapors formed by theevaporating SiC source interact with the material of the tantalumcontainer. As a result of this interaction, a low-melting-point tantalumsilicon alloy is formed which can lead to the destruction of thecontainer at the normal growth temperature.

[0011] In known sublimation techniques for growing SiC single crystals,the vapor source may be either a pre-synthesized SiC powder of thespecified dispersity or a polycrystalline or monocrystalline SiC waferproduced, for example, by the Lely method. Although the use of SiCpowder is more economical than the use of wafers, providing a continuoussupply of powder into the growth zone, as required to grow large singlecrystals, is quite complicated. Additionally, SiC powder often includesimpurities such as graphite or other dust that are transported to thegrowth surface along with the SiC molecules. These impurities lead to ahigh density of micropipes and dislocations in the growing crystal, thussubstantially impacting the crystal quality.

[0012] Accordingly, what is needed in the art is a method and systemthat allows high quality SiC single crystals to be grown. The presentinvention provides such a method and system.

SUMMARY OF THE INVENTION

[0013] The present invention provides a method and apparatus for growinglow dislocation density single crystal silicon carbide. Utilizing thesystem of the invention, silicon carbide can be grown with a dislocationdensity of less than 10⁴ per square centimeter, preferably less than 10³per square centimeter, and more preferably less than 10² per squarecentimeter. The density of micropipes in the as-grown material is lessthan 10 per square centimeter. The density of secondary phase inclusionsis less than 10 per cubic centimeter and preferably less than 1 percubic centimeter. Depending upon the construction of the crucible, theconcentration of tantalum or niobium impurities is less than 10¹⁷ percubic centimeter.

[0014] In accordance with the invention, a SiC source and a SiC seedcrystal of the desired polytype are co-located within a crucible withthe distance separating the source evaporating surface from the growingsurface being comparable to the track length of a SiC molecule. Thegrowth zone is defined by the substantially parallel surfaces of thesource and the seed in combination with the sidewalls of the crucible.Prior to reaching the growth temperature, the crucible is evacuated andsealed, either directly or through the use of a secondary containerhousing the crucible.

[0015] In further accordance with the invention, the crucible iscomprised of tantalum or niobium that has been specially treated. As aresult of the treatment, the inner surfaces of the crucible exhibit adepth variable composition of Ta—Si—C or Nb—Si—C that is no longercapable of absorbing SiC vapors as the monocrystalline silicon carbideis grown. Accordingly, during crystal growth the vapor-phase compositionwithin the crucible is close to the SiC—Si system with the partialpressure of Si-vapor slightly higher than that in the SiC—Si system.Additionally, the resultant Ta—Si—C or Nb—Si—C material is refractory,thus allowing it to withstand the operating temperatures required togrow the silicon carbide.

[0016] The crucible is initially fabricated from tantalum or niobiumthat is preferably at least 99.9 percent pure. Once the crucible isshaped, it undergoes a series of processing steps to clean the surfacesand remove surface contaminants. A thin, near-surface layer of Ta—C orNb—C is then formed and annealed, resulting in a surface that will notinteract with carbon particles. Lastly the crucible is annealed insilicon containing vapor that is diluted by an inert gas, preferablyargon, resulting in the formation of a depth variable composition ofTa—Si—C or Nb—Si—C on the crucible surfaces.

[0017] A further understanding of the nature and advantages of thepresent invention may be realized by reference to the remaining portionsof the specification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a cross-sectional view of a crucible according to theinvention;

[0019]FIG. 2 is a cross-sectional view of a crucible similar to thatshown in FIG. 1 with multiple silicon carbide seeds contained therein;and

[0020]FIG. 3 is a cross-sectional view of a crucible similar to thatshown in FIG. 1 with a separate, sealable container.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS Growth Process

[0021] According to the invention and as illustrated in FIG. 1, asilicon carbide (SiC) source 101 and a SiC seed crystal 103 of thedesired polytype (e.g., 4H, 6H, 3C, etc.) are co-located within acrucible 105. An axial growth zone 107 is defined by the substantiallyparallel surfaces of source 101 and seed 103 in combination withsidewalls 109 of crucible 105. If multiple seed crystals 103 are used asillustrated in FIG. 2, their growth surfaces are located within the sameplane and parallel to the evaporating surface of source 101.

[0022] The distance between the evaporating surface of source 101 andthe growing surface of seed crystal 103 is preferably not much in excessof the track length of a SiC molecule. This configuration enhances thecrystal growth rate as the precipitation of source vapors outside of theseed crystal growth surface is minimized.

[0023] In the preferred embodiment, source 101 is comprised of SiCceramics that are produced from SiC powder that has been sintered at atemperature that permits partial over-sublimation of the SiC grains. Thesintering process is preferably carried out in an inert gas environment(e.g., argon) within a temperature range of 1500 to 2300° C. In additionto achieving partial binding of the powder, during the sintering processbasic background impurities and dust are removed from the powder, thuspreventing the dust composition from being transferred from theevaporating surface of source 101 in the vapor phase. The SiC ceramicsused for source 101 can also be fabricated by compressing SiC powder.

[0024] Additionally, during the fabrication of the SiC ceramics, adoping agent can be deliberately introduced. By using SiC ceramics inwhich the dopant has been uniformly distributed throughout the entirevolume, a uniformly doped single crystal can be grown.

[0025] In another embodiment of the invention, a SiC poly-crystal ormono-crystal source is used.

[0026] As shown in FIG. 1, in the preferred embodiment SiC source 101 islocated on the bottom of crucible 105 and seed crystal 103 is mounted tothe top surface of the crucible. Alternately, source 101 can be mountedon the top surface of the crucible with seed crystal 103 mounted to thecrucible's bottom surface. In order to prevent the loss of SiC sourcematerial due to precipitation of SiC vapors outside the growth surface,preferably the inner dimensions of crucible 105 in general, and cruciblesidewalls 109 in particular, do not exceed the dimensions of growth zone107. Accordingly, in this embodiment of the invention, sidewalls 109define the periphery of growth zone 107. If the inner dimensions ofsidewalls 109 do exceed the dimensions of growth zone 107, preferably itis by a minor amount.

[0027] As described in further detail below, crucible 105 is comprisedof tantalum (Ta) or niobium (Nb) that has been specially treated. It hasbeen established by the inventors that as a result of such treatment,the crucible exhibits a depth-variable composition of Ta—Si—C or Nb—Si—Cthat is no longer capable of absorbing SiC vapors as the monocrystallinesilicon carbide is grown. Consequently, during crystal growth thevapor-phase composition within crucible 105 is close to the SiC—Sisystem with the partial pressure of Si-vapor slightly higher than thatin the SiC—Si system. Additionally, the resultant variable compositionTa—Si—C or Nb—Si—C is a refractory material that can withstand theoperating temperatures required to grow the SiC single crystal.

[0028] Preferably crucible 105 is capable of being evacuated and sealed,either directly or indirectly through the use of an external container301 as illustrated in FIG. 3. In one embodiment of the invention, aftercrucible 105 is loaded with source 101 and seed 103, it is placed withina high temperature oven 111. Oven 111 provides an axial temperaturegradient from seed 103 to source 101, resulting in the evaporation ofthe SiC of source 101 and vapor phase crystallization of SiC on thegrowing surface of seed 103. In this embodiment crucible 105 (orcontainer 301) is sealed before the final operating temperature isreached, sealing being accomplished using any of a variety of sealingsystems (e.g., vacuum welding, graphite or other based sealants, etc.).In an alternate embodiment, crucible 105 (or container 301) is evacuatedand hermetically sealed prior to placement within high temperaturefurnace 111.

[0029] If the crystal growing process is run for an extended timeperiod, for example as required to grow an exceptionally large crystal,the gradually increasing thickness of the grown crystal is accompaniedby a corresponding decrease in the thickness of source 101. Accordingly,in order to maintain the growth process, a large source must be used,for example, a source rod of SiC that can be continuously fed intogrowth zone 107.

[0030] In the illustrated embodiment, furnace 111 provides the requiredthermal gradient, either through the use of multiple temperature zones(e.g., one zone for source 101 and one zone for seed crystal 103) orother means. During crystal growth, a stable temperature profile must bemaintained throughout the entire growth period. Preferably this isachieved by altering the relative positions of crucible 105 and furnace111, for example by moving crucible 105 within furnace 111 at a ratethat is substantially equivalent to the growth rate.

[0031] As previously disclosed, preferably growth zone 107 is evacuatedand sealed prior to initiation of the sublimation process. As a result,material losses from source 101 are substantially reduced. Additionally,sealing crucible 105, either directly or through the use of separatecontainer 301, prevents foreign impurities from the environment fromentering into growth zone 107.

[0032] By isolating the growth zone from the environment and using acrucible exhibiting a depth-variable composition of Ta—Si—C or Nb—Si—C,the vapor phase in the growth zone shifts from the SiC—C system to theSiC—Si system. Furthermore, as the depth-variable composition of Ta—Si—Cor Nb—Si—C remains relatively unchanged for an extended period of time,a stable composition of the vapor phase within growth zone 107 can beachieved, thereby resulting in improvements in both crystal quality andsize. For example, the present invention allows SiC single crystals tobe grown with a dislocation density of less than 10⁴ per squarecentimeter, preferably less than 10³ per square centimeter, and morepreferably less than 10² per square centimeter. The density ofmicropipes in the as-grown material is less than 10 per squarecentimeter. The density of secondary phase inclusions is less than 10per cubic centimeter and preferably less than 1 per cubic centimeter.Depending upon the construction of the crucible, the concentration oftantalum or niobium impurities is less than 10¹⁷ per cubic centimeterand typically in the range of 10¹⁶ to 10¹⁷ per cubic centimeter.

[0033] In order to grow a SiC crystal of the 4H polytype, a 4H polytypesingle crystal is used as seed 103 and the growth process is carried outin the presence of tin vapors that have been introduced into thecrucible prior to its sealing.

Crucible Preparation

[0034] As previously disclosed, crucible 105 is preferably fabricatedfrom tantalum or niobium, thereby allowing the vapor composition duringgrowth to remain close to equilibrium thus reducing crystalgraphitization. The tantalum or niobium must be pre-treated, however, toprevent the metals from interacting with the Si—C vapor and forminglow-temperature eutectics. The pre-treatment consists of two stages.First, tantalum or niobium carbide layers are formed on the surface.During SiC crystal growth, binding of carbon vapor leads to deeperdiffusion of carbon atoms into the pure metal. Second, tantalum orniobium silicide surface layers are formed, the existence of whichincreases silicon containing species partial pressure.

[0035] Initially crucible 105 is fabricated from metallic tantalum orniobium, the metal being at least 99.9 percent pure and of any suitableshape (e.g., rod, rolled, etc.). It is understood that the shape ofcrucible 105 is not limited to the shape shown in FIGS. 1-3. Once thecrucible has been shaped, for example using standard machiningprocesses, it is initially cleaned with standard organic solvents. Thecrucible is then boiled for 30 minutes in a pre-heated acid solutioncomprised of a 3:1 mixture of HCl and HNO₃, the acid solution removingmetallic remnants left on the surface after crucible shaping. Thecrucible is then etched in a room temperature 1:1 mixture of HNO₃ and HFfor approximately 20 to 30 seconds. This etching step must be short toinsure that the etchant does not damage the surface finish quality ofthe crucible. After etching, the crucible is washed in distilled ordeionized boiling water for 10 minutes, the water being changed threetimes during the process. Lastly, the crucible is dried.

[0036] After the crucible has been fabricated and the surfaces cleaned,preferably following the above-described process, the crucible isprocessed in carbon containing vapor in order to form a thin,near-surface layer of Ta—C or Nb—C. After further temperatureprocessing, this layer protects the crucible's surfaces from interactionwith carbon particles.

[0037] During carbon vapor processing, the crucible is placed within afurnace and annealed at a pressure of at least 10⁻³ Torr in graphitethat is at least 99.99 percent pure. Alternately, the crucible isannealed at a pressure of at least 10⁻¹ Torr in vapor that containscarbon species. Preferably a step-wise annealing process is used, suchthat the crucible is annealed at 800°±50° C. for one hour, followed by a1500°±50° C. anneal for one hour, and ending with a 2000°±50° C. annealfor two hours. As a result of this annealing process, a thinnear-surface layer comprised of Ta—C or Nb—C carbides is formed on theentire surface of the crucible. The depth of carbon penetration forcrucibles in which the material is prepared by vacuum melting or metalrolling is approximately 5 to 30 microns while the penetration depth forcrucibles formed of materials prepared by powder metallurgy isapproximately 100 to 500 microns, the greater penetration depth due toaccelerated diffusion along grain boundaries. Quality of the layer aswell as the boundary between the metal and the carbide is governed bythe technique used to fabricate the metal (e.g., powder metallurgy,rolled metal, vacuum melting, etc.).

[0038] After formation of the carbide layer, the crucible is subjectedto further temperature processing, thus assuring that the layer protectsthe surface of the crucible from interaction with carbon particles.During this processing step the crucible is placed in graphite powder inan argon atmosphere, the graphite powder being at least 99.99 percentpure with a grain size of less than 100 microns and the argon being atleast 99.999 percent pure. If the crucible is formed of tantalum it isannealed at a temperature of between 2200 and 2500° C. If the crucibleis formed of niobium it is annealed at a temperature of between 2000 and2300° C. The annealing time for either material is at least 2 hours, thefinal annealing time being governed by the thickness of the crucible.

[0039] Lastly the crucible is annealed for at least 2 hours in siliconcontaining vapor (i.e., SiC or Si vapor) that is diluted by purifiedargon. The annealing temperature for a crucible formed of tantalum isbetween 2200 and 2500° C. while the annealing temperature for a crucibleformed of niobium is between 2000 and 2300° C. During annealing, thesource of Si-vapor should not come into contact with the crucible. Thus,for example, SiC powder is located in a higher temperature zone than thecrucible with a temperature difference of between 10 and 20° C.

[0040] After completion of each of the annealing steps outlined above,the crucible is cooled to room temperature at a cooling rate less than20° C. per minute. Additionally, during each annealing step thetemperature variation across the surface of the crucible should be lessthan 20° C. After all stages of annealing are complete, the amount ofcarbon that has penetrated into the tantalum or niobium cruciblesurfaces should be more than 0.02 grams per square centimeter while theamount of silicon that has penetrated into the tantalum or niobiumcrucible surfaces should be more than 0.0005 grams per squarecentimeter.

EXAMPLES

[0041] SiC crystals have been grown using the disclosed processes,examples of which are provided below. A pre-treated tantalum cruciblewas used. During growth, the evaporating surface of source 101 wasseparated from the growing surface of seed 103 by approximately 3 to 18millimeters. An operating temperature of between 1800-2500° C. was usedwith an axial temperature gradient of between 20 and 25 degrees percentimeter, yielding a growth rate of between 0.7 and 1.2 millimetersper hour.

[0042] Seed crystal polytypes included 6H SiC growing in direction[0001] Si; 4H SiC growing in direction [0001] C; and 6H SiC growing in adirection lying at an angle of 5 degrees to direction [0001].

[0043] The as-grown single crystals were approximately 10 millimetersthick with diameters ranging between 20 and 75 millimeters. The densityof dislocations was in the range of 10² and 10⁴ per square centimeter,the density being dependent upon the doping. The density of micropipeswas less than 10 per square centimeter while the density ofsecondary-phase inclusions (i.e., carbon and silicon) was 10 per cubiccentimeter. The measured concentration of background impurities was 10¹⁶per cubic centimeter for nitrogen; 10¹⁶ per cubic centimeter for boron;and 10¹⁶ to 10¹⁷ per cubic centimeter for tantalum. There was nomeasurable graphitization of the source or the growing surface during a10 hour growth procedure which was performed at a temperature of 2000°C. and at a pressure of 10⁻⁵ Torr. The seed polytype reproducibility was80 percent for a 6H SiC seed growing in direction [0001] Si; 70 percentfor a 4H SiC seed growing in direction [0001] C; and 100 percent for a6H SiC seed growing in a direction lying at an angle of 5 degrees todirection [0001].

[0044] As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

What is claimed is:
 1. A method of producing a silicon carbide singlecrystal, the method comprising the steps of: selecting a tantalumcrucible with an inner surface comprised of a depth variable compositionof Ta—Si—C; locating a silicon carbide source within said tantalumcrucible; locating at least one seed crystal within said tantalumcrucible, wherein a growing surface of said at least one seed crystal issubstantially parallel to an evaporating surface of said silicon carbidesource; sealing said tantalum crucible prior to initiating a sublimationgrowth process, wherein said sealed tantalum crucible is substantiallyvacuum tight; heating said tantalum crucible, wherein said heating stepforms an axial temperature gradient in a direction from said at leastone seed crystal to said silicon carbide source; and growing the siliconcarbide single crystal by said sublimation growth process within agrowth zone of said tantalum crucible.
 2. The method of claim 1, furthercomprising the step of substantially restricting a growth zone volume toa volume defined by said growing surface of said at least one seedcrystal, by said evaporating surface of said silicon carbide source, andby said inner surface of said tantalum crucible.
 3. The method of claim1, further comprising the step of limiting a volume corresponding tosaid tantalum crucible by a plane of attachment of said at least oneseed crystal and by a periphery corresponding to said silicon carbidesource.
 4. The method of claim 1, further comprising the step oflocating a plurality of tantalum crucible interior side walls proximateto said growth zone.
 5. The method of claim 1, further comprising thestep of selecting a silicon carbide ceramic as said silicon carbidesource.
 6. The method of claim 1, further comprising the step ofpreparing said silicon carbide source, wherein said preparing step iscomprised of the step of sintering a silicon carbide ceramic powder at atemperature providing partial oversublimation of a plurality of siliconcarbide grains.
 7. The method of claim 6, wherein said temperature isbetween 1500° C. and 2300° C.
 8. The method of claim 6, wherein saidsintering step is performed in an inert atmosphere.
 9. The method ofclaim 1, said sealing step further comprising the steps of: enclosingsaid tantalum crucible within a secondary sealing container; and sealingsaid secondary sealing container.
 10. The method of claim 9, whereinsaid secondary sealing container is a graphite sealing container. 11.The method of claim 1, said sealing step further comprising the step ofvacuum welding at least one tantalum crucible joint.
 12. The method ofclaim 1, wherein said axial temperature gradient is between 20 and 25degrees per centimeter.
 13. The method of claim 1, further comprisingthe step of separating said growing surface of said at least one seedcrystal and said evaporating surface of said silicon carbide source bybetween 3 and 18 millimeters.
 14. The method of claim 1, furthercomprising the step of selecting said seed crystal from the groupconsisting of 4H, 6H, and 3C.
 15. The method of claim 1, furthercomprising the step of evacuating said tantalum crucible prior toperforming said sealing step.
 16. The method of claim 1, furthercomprising the step of maintaining said axial temperature gradientthroughout said sublimation growth process.
 17. The method of claim 1,further comprising the step of introducing tin vapors into said growthzone prior to performing said sealing step.
 18. The method of claim 1,wherein said heating step achieves an operating temperature of between1800° C. and 2500° C.
 19. A method of producing a silicon carbide singlecrystal, the method comprising the steps of: selecting a niobiumcrucible with an inner surface comprised of a depth variable compositionof Nb—Si—C; locating a silicon carbide source within said niobiumcrucible; locating at least one seed crystal within said niobiumcrucible, wherein a growing surface of said at least one seed crystal issubstantially parallel to an evaporating surface of said silicon carbidesource; sealing said niobium crucible prior to initiating a sublimationgrowth process, wherein said sealed niobium crucible is substantiallyvacuum tight; heating said niobium crucible, wherein said heating stepforms an axial temperature gradient in a direction from said at leastone seed crystal to said silicon carbide source; and growing the siliconcarbide single crystal by said sublimation growth process within agrowth zone of said niobium crucible.
 20. The method of claim 19,further comprising the step of substantially restricting a growth zonevolume to a volume defined by said growing surface of said at least oneseed crystal, by said evaporating surface of said silicon carbidesource, and by said inner surface of said niobium crucible.
 21. Themethod of claim 19, further comprising the step of limiting a volumecorresponding to said niobium crucible by a plane of attachment of saidat least one seed crystal and by a periphery corresponding to saidsilicon carbide source.
 22. The method of claim 19, further comprisingthe step of locating a plurality of niobium crucible interior side wallsproximate to said growth zone.
 23. The method of claim 19, furthercomprising the step of selecting a silicon carbide ceramic as saidsilicon carbide source.
 24. The method of claim 19, further comprisingthe step of preparing said silicon carbide source, wherein saidpreparing step is comprised of the step of sintering a silicon carbideceramic powder at a temperature providing partial oversublimation of aplurality of silicon carbide grains.
 25. The method of claim 24, whereinsaid temperature is between 1500° C. and 2300° C.
 26. The method ofclaim 24, wherein said sintering step is performed in an inertatmosphere.
 27. The method of claim 19, said sealing step furthercomprising the steps of: enclosing said niobium crucible within asecondary sealing container; and sealing said secondary sealingcontainer.
 28. The method of claim 9, wherein said secondary sealingcontainer is a graphite sealing container.
 29. The method of claim 19,said sealing step further comprising the step of vacuum welding at leastone niobium crucible joint.
 30. The method of claim 19, wherein saidaxial temperature gradient is between 20 and 25 degrees per centimeter.31. The method of claim 19, further comprising the step of separatingsaid growing surface of said at least one seed crystal and saidevaporating surface of said silicon carbide source by between 3 and 18millimeters.
 32. The method of claim 19, further comprising the step ofselecting said seed crystal from the group consisting of 4H, 6H, and 3C.33. The method of claim 19, further comprising the step of evacuatingsaid niobium crucible prior to performing said sealing step.
 34. Themethod of claim 19, further comprising the step of maintaining saidaxial temperature gradient throughout said sublimation growth process.35. The method of claim 19, further comprising the step of introducingtin vapors into said growth zone prior to performing said sealing step.36. The method of claim 19, wherein said heating step achieves anoperating temperature of between 1800° C. and 2500° C.